Steam turbine stop valves are employed as the last line of defense against overspeed and thus, must be completely reliable. Any excessive friction or reduced velocity imposed by the valve opening/closing actuator is unacceptable since required closing times are on the order of 0.2 seconds.
Previous approaches to this problem have focused on the reliability of the valve actuating system per se. High pressure hydraulic cylinders combined with quick-opening dump valves have been employed on the majority of existing turbine stop valves. While such arrangements have been very reliable, they require remote hydraulic power units as well as fluid treatment systems and field installed hydraulic tubing. Other systems employ non-hydraulic actuators in order to eliminate the complication and expense of the hydraulic systems, but actuators of this type to date generally have experienced friction and inertia problems sufficient to slow valve closure beyond acceptable limits.